This invention relates to normally tacky and pressure-sensitive adhesive tape and is particularly concerned with a novel composite pressure-sensitive adhesive layer.
Users of pressure-sensitive adhesive tape have frequently sought a product which possesses the combined characteristics of (1) good adhesion to irregular substrates and (2) good resistance to failure when subject to shearing forces after application. These two desiderata are essentially mutually exclusive, since a soft adhesive is required for conformability but a hard, or firm, adhesive is required for good shear properties. Thus, while adhesives possessing intermediate properties have been made (cf., e.g., U.S. Pat. No. 2,956,904, which teaches the use of an electron beam to crosslink an adhesive, and U.S. Pat. No. 3,658,740 which describes the blending of a tackifier with two types of rubbery copolymer), it has generally been considered impossible to obtain a tape in which the adhesive simultaneously possesses both good conformability and good shear properties.
One application for normally tacky and pressure-sensitive adhesive tape which imposes the conflicting demands just discussed is found in tape used for diaper closures. Such tape must bond quickly and firmly to the irregular surfaces of conventional cloth diapers or to the frequently powder-contaminated polyethylene outer face of disposable diapers. On the other hand, the stresses imposed on a tape product used as a diaper closure are such that almost all adhesives which are considered "soft" (a characteristic which is required for conformability and contamination tolerance) quickly fail in use because of the shearing action applied to the adhesive layer.
Normally tacky and pressure-sensitive adhesives of certain water-resistant types have also been used in the manufacture of identification labels for shirts and other launderable garments; see, e.g., U.S. Pat. No. 4,074,004. Again, while softness of the pressure-sensitive adhesive is essential to obtain conformability to the cloth, the stresses imparted during washing-drying cycles tend to loosen the edges of a fabric label having a soft adhesive and hence to result in a shorter life for the identification label than is desirable. A hard adhesive, on the other hand, lacks sufficient conformability to develop adequate initial adhesion and hence peels off readily. It has been extremely difficult to balance adhesive properties to arrive at a label which not only adheres well but also resists the shearing stress encountered in normal laundering.